The invention relates to DC restore circuits and to sync decode circuits that operate on composite video signals, and particularly to improvements in such circuits to avoid distortion associated with prior DC restore circuits and to produce horizontal and vertical sync pulses more accurately than prior sync decoder circuits by avoiding signal jitter associated therewith.
Composite video signals typically include a considerable amount of AC noise. Such AC noise can make it very difficult to separate the horizontal and vertical synchronization information needed to produce a CRT (cathode ray tube) video display, and also can make it difficult to generate a DC restored composite video signal, horizontal sync pulses, and vertical sync pulses as accurately as is desirable for high resolution screen displays, especially if the video signal is weak. Those skilled in the art know that composite video signals lose their reference voltage levels when they are transmitted through space to a receiver, or are recovered from magnetic media by means of a read head during playback. A stable reference must be re-created by a technique referred to as DC restoration. Some prior DC restoration circuits capacitively couple the composite video signal to a diode clamping circuit having its anode connected to ground and its cathode connected to one terminal of the coupling capacitor. In such DC restore circuits, the sync tip gets clamped to a voltage equal to one diode drop below ground, thereby establishing or restoring a reference level to the sync tip. This technique works well for non-critical applications in which there is a reasonably strong video signal level and not too much noise. However, shifts in the noise level, in the video signal strength, or in the sync tip amplitude can result in severe distortions in the DC restored signal produced by such prior circuits. Another prior circuit is referred to as a "back porch clamp" or a "sync tip clamp". This type of circuit, although more complex than the one first described, and less subject to variations in the sync tip amplitude, typically is based on diode clamping to establish the sync tip level, and produces severe distortions in the resulting DC restored signal when high levels of noise are present relative to the video signal. There remains an unfulfilled need for an improved technique for producing a DC restored signal which is a precise replica of the composite video waveform.
Prior art sync decoders typically apply the stripped video signal, i.e., the composite sync signal to an RC integrating circuit in the process of producing the vertical sync pulse, and apply the same composite sync signal to an RC differentiating circuit to produce the horizontal sync pulse signal. The resulting integrated and differentiated signals then are used to trigger Schmitt trigger circuits. The presence of noise on the composite sync waveform produces variations in the slopes of the integrated and differentiated waveforms, producing "jitter" of the times at which the Schmitt trigger circuits are triggered, and thereby producing "jitter" (with respect to time) of the horizontal and vertical sync pulses, especially the latter. The jitter usually is unacceptable for applications in which high resolution images are required, or if images are to be digitized stored, and further processed. Therefore, there remains an unmet need for an improved technique for generating jitter-free horizontal and vertical sync pulses from a composite video waveform.